Arrays of photo-sensors, typically arrays of photo diodes, are well known and in wide use for many applications, such as the capture of images by digital cameras and digital video recorders. When an image is focused by a lens onto such an array, it is often referred to as a Focal Planar Array, or “FPA.” The term “FPA” is used herein to refer generically to any array of photo sensors, unless otherwise required by the context.
FPA's take on many configurations, according to constraints such as cost, bandwidth of light to be sensed/imaged, required image capture speed, resolution requirements, and required sensitivity. Typically, each photo-sensor or “pixel” in an FPA provides output to a preamplifier of some kind. For applications where image capture speed is not critical and maximum sensitivity is needed, an integrating preamplifier is often used. An integrating preamplifier includes a capacitor that accumulates charge during a sampling period, so that the preamplifier provides an output having a signal-to-noise ratio that is improved according to the square root of the sampling period. An integrating preamplifier also includes a reset that is configured to discharge the capacitor and begin a new measurement at the end of each sampling period.
For some applications, an FPA may be susceptible to very large optical exposures that can at least temporarily blind photo-sensors and/or the optical apparatus as a whole. When a photo-sensor is exposed to an excess of light, it is unavoidable that it will be saturated and temporarily blinded. Once the excess light is removed, a “recovery time” will be needed before the photo-sensor can provide an undistorted output. While the recovery time is not necessarily a linear function of the saturating light intensity, it is generally true that the recovery time of the preamplifier will be longer for stronger saturating light intensities.
In cases where only one or only some of the photo-sensors in an array are subject to a saturating light exposure, it can sometimes happen that surrounding photo-sensors that are not directly exposed to the saturating light will nevertheless be saturated. The apparent result in an image is a spreading out of a white “blind spot” from the affected pixel(s) to surrounding pixels. This effect is often referred to as “blooming.” This problem can be especially problematic when the preamplifiers are integrating preamplifiers.
A circuit for a typical prior art integrating preamplifier is shown in FIG. 1. The preamplifier is basically a cascode amplifier, including an input FET 102 and a cascode FET 104. The drain of the cascode FET is fed by a current source FET 106. The input of the preamplifier 100 is current from a photodiode. Also, through the feedback action of the preamplifier, an equal and opposite current is fed to the preamplifier input 100 through capacitor 108 so as to hold the amplifier input voltage constant. For this reason, the input 100 is sometimes referred to as a “summing junction.” The effect of this current is to charge capacitor 108, causing the output 110 of the preamplifier to represent a result that is summed or “integrated” over some sampling period. When the sampling period is ended, the capacitor is discharged by a reset FET 112, and a new sampling period begins.
At least one set of circumstances that can lead to blooming in an apparatus such as FIG. 1 is as follows. Excessive irradiation of the photodiode can cause a high current from the photodiode output, which is the preamplifier input 100, causing the preamplifier input voltage to rise. If this voltage rises high enough, it can cause the photodiode to become forward biased, so that the diode begins to freely conduct current in a manner similar to a conventional forward-biased diode. Since the voltage supply for the photodiode is designed to support the normally small diode photocurrent, this sudden jump in current can exceed the voltage supply's current capacity, thereby causing the supply's output voltage to change.
Typically, many or all of the photo-sensors in an array will be supported by a single biasing voltage supply. Hence, if a saturated photo-sensor in the array becomes forward-biased and causes the biasing voltage supply to change, this will affect not only the saturated photo-sensor, but also all of the other photo-sensors that are supported by that same biasing supply, causing a saturated “white spot” in a resulting image to “bloom” outward from the saturated pixels to other, surrounding pixels that are not directly saturated. As a result, a significant portion of the image can be lost.
Of course, the preamplifiers associated with any such “bloomed” pixels will also require a recovery time after the saturating light is removed, before they can once again provide undistorted output.
With reference to FIG. 2, one approach to reducing recovery time and limiting or preventing blooming is to connect a separate clamping transistor 200 to the photodiode output 100, and to bias the clamping transistor 200 with an anti-blooming reference voltage 202 that is adjusted such that the clamping transistor 200 begins to conduct when the summing junction voltage rises above some set level. The other side of the clamping transistor 200 can be connected to the amplifier output, or to ground as shown in the figure.
This approach is basically a “brute force” method for limiting the maximum photodiode output voltage, which attempts to directly limit or clip the extent of unwanted photodiode output voltage rise. Unfortunately, adding circuitry to limit the photodiode output voltage rise to as small a value as possible, while not appreciably affecting preamplifier speed or noise performance, and by using as simple a circuit as possible, are inherently conflicting requirements that necessarily entail a performance compromise. For a simple circuit such as is illustrated in FIG. 2, the clamping transistor 200 will not immediately switch to full conduction when the threshold input voltage is reached. Instead, it will typically increase its conductance over some range of input voltages, and may never fully clamp the photodiode output voltage. The on/off behavior of the clamping circuit can be improved by increasing its complexity. However, this can lead to added cost, added power consumption, and added space requirements.
The photodiode output voltage under normal (non-overload) operating conditions varies by only a few tens of millivolts. For a circuit such as FIG. 2, setting the anti-blooming threshold high enough to avoid compromising preamplifier performance generally permits a photodiode output voltage rise in the hundreds of millivolts before substantial clipping takes place. While this may prevent blooming with moderate overloads, the voltage increase is likely large enough to cause significant integrating amplifier operating point changes, with a possible long recovery time following the removal of the overload. For more severe overloads, the clamping ability of this circuit can easily be exceeded.
Thus the degree of overload and anti-blooming protection afforded by prior art clamping circuits such as the one shown in FIG. 2 is, at best, limited and uncertain. In effect, this approach does not prevent preamplifier overload, but functions only as an overload damage-control circuit that attempts to limit the propagation of deleterious overload effects to other photo-sensor circuits, and which in some cases is largely ineffective.
Another approach to limiting the input voltage for an overloaded integrating preamplifier is to switch the amplifier into reset, so as to restrain the rise in photodiode output voltage. However, this approach may require the preamplifier output drive capability to exceed the maximum overload current delivered to the preamplifier input, a condition that the preamplifier may not be able to meet, at least not without an increase in preamplifier complexity.
What is needed, therefore, is an apparatus that can sharply clamp the input to a photo-sensor preamplifier when the input only minimally exceeds the normal operating range of the photo-sensor, thereby minimizing the recovery time of the preamplifier and eliminating blooming caused by overloading of current supplies.